Do you think that a decreased sex life means a relationship/marriage will fail?

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Goodness no... sex is one slice of the pie of a healthy relationship..  if you have good communication, happiness and fulfillment then that's all you truly need... some people put HUGE emphasis on sex in a relationship but truely there are so many factors... how they deal with stress, family, work, finances, balance, health..  comparability is beyond sex..

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For me, yes. But everyone is different.  Sex is a huge part of my relationship and communication. Lack of sex leads to more frustration and tension in my relationship.

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Not at all! There is so much more to a strong relationship other than sex.

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If the sex drive of both parties fade, it can work. If it’s one sided, the relationship will likely fail.

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No it doesn't have to be, but both people have to be understanding towards one another. 
Your sex life is inevitably going to change in a long term relationship, probably a lot of times and go through phases.
Personally, I tend to be pretty hypersexual, and sex helps me feel or express feeling connected to my partner... so it is difficult for me to make sense of if my sex life changes in a relationship, especially suddenly. All I need is to be able to understand what is going on, and I can be calm about it. If it's an unknown problem it really worries me. If the reason for less sex is obvious, I won't even worry. (Say my partner is having health problems, had surgery, changed meds and is having withdrawal symptoms, is going through a tough time emotionally, etc.) 
But no cuddling? I don't think I can do that.

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In the end if you are both satisfied sexually then the amount doesnt matter. But if one of you has a complaint about the frequency or quality, then yes it puts a strain.

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Nope. My husband and I used to have tons of sex. 4ish years intense stopped completely for 3 years (for reasons I wont get into here). We had a solid marriage and were plenty intimate just not sexually. This past april we started have sex again and it's been amazing.